A light-emitting element using an organic compound as a luminous body, which has features such as thinness, lightness, high-speed response, and DC drive at low voltage, is expected to be applied to a next-generation flat panel display. In particular, a display device in which light-emitting elements are arranged in a matrix is considered to have advantages in a wide viewing angle and excellent visibility over a conventional liquid crystal display device.
A light-emitting element is said to have the following light emission mechanism: when voltage is applied between a pair of electrodes with an EL layer containing a light-emitting substance provided therebetween, electrons injected from a cathode and holes injected from an anode are excited in a light emission center of the EL layer, and energy is released and light is emitted when the excited state returns to a ground state. There can be two types of the excited states generated in the case of using an organic compound as a light-emitting substance: a singlet excited state and a triplet excited state. Luminescence from the singlet excited state (S1) is referred to as fluorescence, and luminescence from the triplet excited state (T1) is referred to as phosphorescence. The statistical generation ratio of the excited states in the light-emitting element is considered to be S1:T1=1:3.
Development for improving element characteristics has been conducted; for example, a light-emitting element having a structure utilizing not only fluorescence but also phosphorescence has been developed. In a light-emitting layer of the light-emitting element, a host material and a guest material are contained, and a phosphorescent material exhibiting high energy emission is used as the guest material (e.g., see Patent Document 1).